It is well known that the stiffness and strength of a polymer are related to the flexibility of the polymer chain on the molecular level. Thus, if the chemical structure of the main chain restricts chain coiling and flexing, the resulting polymer will be stiff and strong. An example of a stiff polymer-is poly-1,4-phenylene-1,4-terephthalamide (PPTA). While PPTA can coil in solution, the amide linkages and para-phenylene groups favor an extended chain conformation. Fibers can be prepared in which the chains are essentially all extended into rod-like conformations, and these fibers are extraordinarily strong and stiff. Unfortunately, PPTA is difficult to process (except for fiber spinning) and cannot be molded or extruded. In general, the more rigid the polymer main chain the more difficult it is to prepare and process.
Some applications require strong, stiff materials that can be easily processed by molding or extrusion. A widely used approach to obtain such stiff materials is to add fillers such as carbon or silica, or to incorporate fibers, such as glass and carbon fibers, into a relatively flexible polymer, thereby forming a stiff, strong composite material. The most widely utilized, high-performance fiber-polymer composites are composed of oriented carbon (graphite) fibers embedded in a suitable polymer matrix.
The improvements in strength and stiffness of composites are related to the aspect ratio of the filler or fiber, i.e., the length to diameter ratio of the smallest diameter cylinder that will enclose the filler or fiber. To contribute reasonable strength and stiffness to the composite, the fibers must have an aspect ratio of at least about 25, and preferably at least 100. Continuous fibers have the highest aspect ratio and yield the best mechanical properties but are costly to process. Low aspect ratio materials, such as chopped fibers and fillers, give limited improvement in mechanical properties, but are easy and inexpensive to process. The success of composites is demonstrated by their wide use as structural materials.
There are several drawbacks associated with composite materials. Composites are often more costly than the unreinforced polymer. This is because of the expense of the fiber component and the additional labor needed to prepare the composite. Composites are difficult or impossible to repair and in general cannot be recycled. Many composites also have undesirable failure characteristics, failing unpredictably and catastrophically.
Molecular composites (composed of polymeric materials only) offer the prospect of high performance, lower cost and easier processability than conventional fiber-polymer composites. In addition, molecular composites generally can be recycled and repaired. Because molecular composites contain no fibers, they can be fabricated much more easily than fiber-polymer compositions, which contain macroscopic fibers.
Molecular composites are materials composed of a rigid-rod polymer embedded in a flexible polymer matrix. The rigid-rod polymer in a molecular composite can be thought of as the microscopic equivalent of the fiber in a fiber-polymer composite. The flexible polymer component of a molecular composite serves to disperse the rigid-rod polymer, preventing bundling of the rigid-rod molecules. As in conventional fiber/resin composites, the flexible polymer in a molecular composite helps to distribute stress along the rigid-rod molecules via elastic deformation of the flexible polymer. Thus, the second, or matrix-resin polymer must be sufficiently flexible to effectively surround the rigid-rod molecules while still being able to stretch upon stress. The flexible and rigid-rod polymers can also interact strongly via Van der Waals, hydrogen bonding, or ionic interactions. The advantages of molecular composites over fiber-based composites are realized by incorporating rigid-rod segments into polymer systems. The advantages of molecular composites have been demonstrated by, e.g., W. F. Hwang, D. R. Wiff, C. L. Benner and T. E. Helminiak, Journal of Macromolecular Science.--Phys., B22, 231-257 (1983).
Molecular composites are simple mixtures or blends of a rigid-rod polymer with a flexible polymer. As is known in the art, most polymers do not mix with other polymers, and attempts at blends lead to macroscopic phase separation. This is also true of rigid-rod polymer/flexible polymer blends. Metastable blends may be prepared by rapid coagulation from solution. However, metastable blends will phase separate on heating, ruling out further thermal processing, such as molding or melt spinning and use at high temperatures. The problem of macroscopic phase separation is reported in H. H. Chuah, T. Kyu, and T. E. Helminiak, Polymer, 28, 2130-2133 (1987). Macroscopic phase separation is a major limitation of molecular composites.
Rigid-rod polymers produced in the past are, in general, highly insoluble (except in the special case of polymers with basic groups, which may be dissolved in strong acids or in organic solvents with the aid of Lewis acids) and infusible. Preparation and processing of such polymers is, accordingly, difficult. A notable exception is found in U.S. patent application Ser. No. 07/397,732, filed Aug. 23, 1989 (assigned to the assignee of the present invention), which is incorporated herein by this reference. The rigid-rod polymers described in the above-referenced application have a rigid-rod backbone comprising a chain length of at least 25 organic monomer units joined together by covalent bonds wherein at least about 95% of the bonds are substantially parallel; and solubilizing organic groups attached to at least 1% of the monomer units. The polymers are prepared in a solvent system that is a solvent for both the monomer starting materials and the rigid-rod polymer product. The preferred monomer units include: paraphenyl, parabiphenyl, paraterphenyl, 2,6-quinoline, 2,6-quinazoline, paraphenylene-2-benzobisthiazole, paraphenylene-2-benzobisoxazole, paraphenylene-2-benzobisimidazole, paraphenylene-1-pyromellitimide, 2,6-naphthylene, 1,4-naphthylene, 1,5-naphthylene, 1,4-anthracenyl, 1,10-anthracenyl, 1,5-anthracenyl, 2,6-anthracenyl, 9,10-anthracenyl, and 2,5-pyridinyl.
The rigid-rod polymers described above can be used as self-reinforced engineering plastics and exhibit physical properties and cost-effectiveness superior to that exhibited by many conventional fiber-containing composites. It would be quite useful if rigid-rod polymers could be incorporated into conventional flexible polymers, especially large volume commodity polymers. The value of a flexible polymer would be increased significantly if its mechanical properties could be enhanced by addition of rigid-rod polymers. Such molecular composites could displace more expensive engineering resins and specialty polymers and conventional composites as well. To date, practical molecular composites have not been demonstrated. This is chiefly due to deficiencies in currently available rigid-rod polymers, namely, limited solubility and fusibility and unfavorable chemical and physical interactions between the rigid-rod and the flexible polymer component.
There is a need in the art for a rigid-rod polymer that can be chemically incorporated into flexible polymers and polymer systems, during or subsequent to polymerization, to thereby add strength and/or stiffness to the resulting polymers. Chemical rather than physical incorporation is desirable to inhibit phase separation during the processing and use of the polymer and to increase the resulting polymer's solvent resistance. The mechanical behavior of polymer systems which contain chemically incorporated rigid-rod moieties can be different and superior to physical blends of, for example, rigid-rod polymers with flexible polymers.